From protogalaxy through thick and thin: Why did the Milky Way evolve in three kinematic phases?
13 affiliations
- 1University of California Department of Physics and Astronomy, , Irvine, CA 92697 ,
- 2University of Southern California Department of Physics and Astronomy, , Los Angeles, CA 90089 ,
- 3The University of Texas at Austin Department of Astronomy, , Austin, TX 78712 ,
- 4Cosmic Frontier Center, The University of Texas at Austin , Austin, TX 78712 ,
- 5Weinberg Institute for Theoretical Physics, The University of Texas at Austin , Austin, TX 78712 ,
- 6Center for Astrophysics | Harvard & Smithsonian , 60 Garden St, Cambridge, MA 02138 ,
- 7Northwestern University Department of Physics and Astronomy and CIERA, , 2145 Sheridan Road, Evanston, IL 60208 ,
- 8University of Zurich Department of Astrophysics, , Winterthurerstrasse 190, 8057 Zurich ,
- 9Yale University Department of Astronomy, , New Haven, CT 06520 ,
- 10Pomona College Department of Physics and Astronomy, , Claremont, CA 91711 ,
- 11Carnegie Observatories , 813 Santa Barbara St., Pasadena, CA 91101 ,
- 12School of Physics and Astronomy, Tel Aviv University , Tel Aviv 69978 ,
- 13University of California Department of Physics and Astronomy, , Davis, CA 95616 ,
Abstract
APOGEE and Gaia data suggest the Milky Way’s kinematic structure evolved through three distinct phases: a disordered protogalaxy, which ‘spun up’ into a thick stellar disk, and then ‘cooled down’ to a final cold, thin stellar disk. We use a suite of FIRE-2 cosmological zoom-in simulations of Milky Way-mass galaxies to demonstrate that the same three phases arise in simulations, and we study their physical origin. In our simulations, the early protogalaxy phase occurs when the rate of cool gas (T ≤ 104 K) converting into stars is low, star formation is bursty, and the baryonic mass ‘sloshes’ within the host potential with respect to the center of mass. The gas begins to spin coherently after sloshing ends, followed by the spin-up of young stars. The central potential is least concentrated just prior to gas spin-up. This second, thick disk phase coincides with the highest rate of cool gas converting into stars, though star formation remains bursty. The final transition to the thin disk phase occurs when the inner circumgalactic medium virializes, and is associated with steady star formation and intermediate consumption rates of cool gas converting into stars. Mergers do not appear to play a defining role in driving transitions between the three phases. The condition for thick disk formation appears to be minimal: a stable center of mass motion. Thin disk formation requires more: gas must accrete slowly enough for its angular momentum to mix and become coherent prior to joining the galaxy.
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BibTeX
@article{myrtaj2026from,
title = {{From protogalaxy through thick and thin: Why did the Milky Way evolve in three kinematic phases?}},
author = {Olti Myrtaj and James S Bullock and Michael Boylan-Kolchin and Vedant Chandra and Claude-André Faucher-Giguère and Robert Feldmann and Pratik J Gandhi and Francisco J Mercado and Jorge Moreno and Jonathan Stern and Andrew Wetzel and Luke Y Xia},
journal = {Monthly Notices of the Royal Astronomical Society},
year = {2026},
month = oct,
doi = {10.1093/mnras/stag1902},
url = {https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stag1902/8907363},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - From protogalaxy through thick and thin: Why did the Milky Way evolve in three kinematic phases?
AU - Olti Myrtaj
AU - James S Bullock
AU - Michael Boylan-Kolchin
AU - Vedant Chandra
AU - Claude-André Faucher-Giguère
AU - Robert Feldmann
AU - Pratik J Gandhi
AU - Francisco J Mercado
AU - Jorge Moreno
AU - Jonathan Stern
AU - Andrew Wetzel
AU - Luke Y Xia
PY - 2026
DA - 2026/10/10
JO - Monthly Notices of the Royal Astronomical Society
DO - 10.1093/mnras/stag1902
UR - https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stag1902/8907363
AB - APOGEE and Gaia data suggest the Milky Way’s kinematic structure evolved through three distinct phases: a disordered protogalaxy, which ‘spun up’ into a thick stellar disk, and then ‘cooled down’ to a final cold, thin stellar disk. We use a suite of FIRE-2 cosmological zoom-in simulations of Milky Way-mass galaxies to demonstrate that the same three phases arise in simulations, and we study their physical origin. In our simulations, the early protogalaxy phase occurs when the rate of cool gas (T ≤ 104 K) converting into stars is low, star formation is bursty, and the baryonic mass ‘sloshes’ within the host potential with respect to the center of mass. The gas begins to spin coherently after sloshing ends, followed by the spin-up of young stars. The central potential is least concentrated just prior to gas spin-up. This second, thick disk phase coincides with the highest rate of cool gas converting into stars, though star formation remains bursty. The final transition to the thin disk phase occurs when the inner circumgalactic medium virializes, and is associated with steady star formation and intermediate consumption rates of cool gas converting into stars. Mergers do not appear to play a defining role in driving transitions between the three phases. The condition for thick disk formation appears to be minimal: a stable center of mass motion. Thin disk formation requires more: gas must accrete slowly enough for its angular momentum to mix and become coherent prior to joining the galaxy.
SN - 0035-8711
SN - 1365-2966
ER -